1. Field of the Invention
The present invention relates to an optical fiber which transmits light emitted from a light source, and further transmits light reflected off of an observed portion. In particular, it relates to a fiber-optics system used in observation equipment for acquiring information on an object.
2. Description of the Related Art
In a fiber-optics system, a single-mode optical fiber, which has a single core and an annular cladding surrounding the core, is utilized to transmit signals to a remote location. The single mode-optical fiber can be used to acquire image information on an object. The core is used for transmitting illuminating light, and the cladding is used for transmitting reflected light. In a double cladding optical fiber, light emitted from a light source passes through the core and exits from the tip portion of the optical fiber. Light reflected off of an object enters into the cladding of the optical fiber and progresses in the opposite direction. The optical fiber can be separated into two fiber segments, between which a beam splitter is provided. Light passing through a core in one optical fiber segment directly enters into a core of the other optical fiber segment via the beam splitter. The reflected light passing through the cladding of the other optical fiber is deflected by the beam splitter, and is directed to a photo-detector, such as a photodiode. The above fiber-optics system can be used in observation equipment, such as an endoscope or probe with a scanning optical fiber, a scanning microscope, and other similar observation systems, including confocal systems.
In the endoscope with a scanning optical fiber, a single-mode optical fiber is provided in the endoscope. The tip portion of the optical fiber is attached to a piezoelectric 2-D actuator such that the tip portion becomes a cantilever beam. The piezoelectric actuator two-dimensionally vibrates the cantilevered tip portion at a resonant frequency while modulating or amplifying amplitudes of the vibration, so that the tip portion of the optical fiber is driven in a spiral pattern. Light emitted from a light source, such as a laser, passes through a core of the optical fiber, and light exiting from the optical fiber is directed to an observed portion. Light reflected off of the observed portion enters into and passes through the cladding of the optical fiber. A photodiode provided in a processor, which is connected to the endoscope, detects the transmitted light.
A part of the reflected light penetrates through the cladding while the reflected light progresses. Consequently, the intensity of the reflected light can not be precisely detected by the photo-detector. On the other hand, in the case of a fiber-optics system with a beam splitter, a part of the light passing through the beam splitter may accidentally or erroneously enter into the cladding due to misalignment of the beam-splitter. Such misalignment may occur due to a change of a temperature or a vibration of the optical fiber. The light which accidentally passes through the cladding toward the tip portion is irradiated toward the observed portion, so that a beam spot on the observed portion is formed by light from the core and light from the cladding. This may result in enlargement of the beam spot.